Wire splices are commonly used in electrical harnesses in the automotive industry, and dimensionally recoverable tubing is often used to protect spliced wires within electrical harnesses in the automotive industry. One of the most common splice configurations is an “inline splice”. In an inline splice, each wire to be spliced has an electrically insulative covering removed (e.g. at one end, or at one or more other locations) to expose bare electrically conductive wire. In some other kinds of splice, removal of an insulative covering is not necessary. The wires to be joined are then arranged as required with all of the exposed bare wires essentially parallel and overlapping each other.
The bare wires are then crimped, welded, soldered or otherwise joined together to form a splice nugget. Subsequently, the nugget and the adjacent exposed conductors must be protected and sealed from the external environment. A preferred means for protecting the nugget and sealing out moisture and other contaminants is to encase the nugget in a dimensionally recoverable tubing, which has a sealant/adhesive interior layer or liner, thereby forming a wire seal. Typically, heat is applied to cause the sealant/adhesive liner to flow, while simultaneously causing the tubing to heat-recover (shrink) about the nugget. The tubing shrinks around the exposed wires and the adhesive/sealant flows within the tubing to cover and seal the exposed wires. The adhesive/sealant also flows along the wires to contact and cover a portion of the unstripped, electrically insulative wire covering. This provides a seal over the entire length of the exposed wires and the splice nugget, up to and including the beginning of the insulative wire covering, and thus prevents water from entering the splice and/or from flowing along the conductors inside the wire insulation. Wire butt splices and wire splices to ring terminals or other termination devices can also be sealed and protected in this way.
In addition, connectors may be sealed against water ingress and bundles of wires blocked using adhesive inserts in combination with heat shrink tubing.
The complexity of vehicle harnesses, and the number of wire splices incorporated in the harnesses, are increasing due to the growing number of electrical functions on modern vehicles. As a result, vehicle manufacturers are using an increasing number of splice sealing products to ensure electrical integrity and guarantee reliability. In order to maximize productivity and minimize cost, it is therefore desirable to minimize the time it take to seal a splice.
A variety of heat-shrinkable splice seal sleeves are commercially available. An example of these is dual-wall heat-shrinkable splice seal sleeve that is a tubular construction having an outer cross-linked polymer heat-shrinkable sleeve in combination with an inner heat-flowable adhesive/sealant liner. When heated, the tubing shrinks and the adhesive/sealant layer melts and flows. Such products are well known in a range of different materials and sizes, and are used in various industries for environmental sealing of cable and wire splices. The products are installed by sliding the sleeve and liner over the area to be sealed and heating using a heat gun, flame, infrared, or other heat source to shrink the tubing. The minimum time taken to achieve a sealed splice depends on a number of factors including the number and size of the component wires that make up the splice, the size of the tube, the recovery temperature of the tube, the melting temperature of the adhesive liner, the viscosity of the liner at the recovery temperature, the hoop stress of the tubing at the recovery temperature, the temperature of the copper nugget, the type of heating device employed and its thermal characteristics.
For convenience herein all such structures are referred to as “splices” and, as the context requires, “splice seals”, although in practice some of the structures to which the invention relates may not require actual splicing together of conductors or other filaments.
It is that known that heat-shrinkable splice seal sleeves are generally either capable of rapid installation or having high temperature resistance.
Rapid installation type splice seal sleeves generally include an adhesive with relatively low viscosity at the installation temperature. This rapid installation characteristic makes them commercially attractive, especially in high-throughput environments. However, the adhesive in such splice seal sleeves typically flows excessively in service at the rated temperature, which renders it unsuitable for high temperature environments. Alternatively, high-temperature resistant splice seal sleeves tend to be manufactured with an adhesive, which has relatively high viscosity at the service temperature, rendering them suitable for use in high temperature environments. However, such an adhesive slows down the rate of product installation, thereby undesirably slowing the production process. In particular, although the manufacturing process is only slowed by a few seconds with high-temperature rated splice seal sleeves, compared to rapid installation ones, when production is carried out on a large scale, as it is in the automotive industry, the extra delay results in a significant costs.
Once installed, the splice seals must meet certain specification requirements, which are designed to reflect the use environment. In the automotive industry, these specifications include sustained sealing during immersion in fuel, temperature cycling and high temperature flow resistance, to reflect the engine compartment environment. To meet these requirements and sustain a seal, the installed adhesive should be resistant to flow at relatively high temperatures. Two of the key requirements for an automotive splice seal are rapid installation and, once installed, minimal adhesive flow in a vertical position at 150° C. The technical solutions to these requirements are in direct conflict—low adhesive viscosity for rapid installation, and yet high viscosity for flow inhibition once installed.
It is known to provide hot-melt adhesives having ethylene vinyl acetate (EVA) copolymers, which copolymers additionally include 1% to 15% fumed silica. The hot melt adhesives are typically used may be used to provide glue sticks which may be used in hot-melt guns. The resultant glue sticks are said in use to be less prone to stringing, and to provide a sag-resistant melt.
U.S. Pat. No. 3,983,070 describes adhesives which are said to be particularly useful in bonding polymeric materials used in encapsulation and termination of insulated electrical conductors. The adhesive includes a polar copolymer of an a olefin and an inorganic silicon-containing compound. The adhesives used in the context of this document are said to be particularly useful for providing the internal coatings of heat-shrinkable sleeves and end caps for cable joints and termination, especially in telephone cables, and preferably have a melt flow index less than 5. The use of the silica-containing adhesive is said to increase the strength of the bonds obtained between cross-linked polyethylene and the lead sheaths of cables, as well as to provide a high peel strength at temperatures as high as 70° C. Telephone cables however typically have dimensions which are substantially greater than those contemplated by the splice seal sleeves of the present invention.
As disclosed in the description, the silicon-containing compound is a chemically treated silica filler, such as Aerosil R972. The resultant adhesive is said to provide a high bond strength when e.g. bonding a polymeric material to another polymeric material, or to another substrate, and also to provide desirable electrical characteristics, and may particularly be used where heat-recoverable materials are used to effect the encapsulation or termination.
There has been a need, especially in high-volume production of wiring harnesses to be installed into cars and other vehicles, for a heat-shrinkable splice seal sleeve capable of rapid installation and having high temperature resistance, because of conflicting requirements for flow characteristics of the adhesive.